The present invention relates to a method of measuring position and orientation with improved signal to noise ratio. The present invention contemplates transmitting a three axis pulsed DC transmit waveform that is received by a three axis receiver. This general concept is well known. However, in systems employing such structure, typically, only the leading edge of the waveform is used in conducting measurements. Typically, the signal to noise ratio is relatively low since only a portion of the signal energy is used.
The concept of using transmitting and receiving components incorporating electromagnetic coupling is well known in the field of bio-mechanics and in minimally invasive surgery. As one example, sensors transmit position information regarding the locations of surgical instruments within the body. This information is employed by a computer and display to precisely show relative motions of instruments so monitored, giving the surgeon valuable information regarding required actions.
Magnetic position and orientation measuring systems are also used in the field of motion capture and digitization in which an actor moves in such a manner as to animate a virtual character. When conductive materials are present adjacent the location where the actor is moving, they generate eddy current fields which distort the received magnetic field signals, thereby causing undesirable errors in the computed sensor position. Systems employing pulsed DC transmit waveforms and various magnetic sensor and signal processing techniques have been developed in an attempt to reduce these negative effects. Sensors employed in these applications measure both the H field and the derivative dH/dT field. The H field is generally measured by a flux gate magnetometer, Hall effect sensor, magneto-optical sensor or magneto-resistive sensor. Calculation of the dH/dT field is typically performed through the use of a coil in series with an integrator.
The following prior art is known to Applicant:
Volume 41, Geophysics, April, 1976, pages 287-299, describe the advantages and disadvantages of DC field (H) measurement systems using a flux gate magnetometer sensor means and measurement of dH/dT using a coil-integrator sensing means when employed in a pulse excited geomagnetic prospecting system. This publication fails to teach or suggest determination of the position of a sensor relative to a transmitter in three dimensions.
U.S. Pat. Nos. 4,849,692 and 4,945,305, both to Blood, disclose a position measuring system in which a pulsed DC waveform is transmitted and the transmitted signal plus eddy field distortion are sensed using a DC responsive sensor. The transmitted waveform is held in a steady state until the eddy current fields decay to a nominal value, at which time the remaining sensed field value is digitized and processed. The systems disclosed in the Blood patents require compensation for the presence of the Earth""s magnetic field, which field is generally an order of magnitude larger than the sensed signal from the transmitter. This requirement adds cost and size to the signal processing system and the requirement to wait until the eddy current field distortion has decayed adds unacceptable additional processing time.
U.S. Pat. No. 4,868,498 to Lusinchi discloses an angular measurement device including a magnetic transmitter element affixed to a rotating body. A transmitted signal is sensed by a coil, the output of which is then integrated to provide a flux reading from the transmitter. The system disclosed by Lusinchi may measure the angular position of a rotating body but is not capable of determining position in three dimensions.
U.S. Pat. No. 5,272,658 to Eulenberg discloses a long-term integrator for integrating a voltage signal from a coil measuring magnetic induction. Eulenberg also discloses the use of a flux measuring coil followed by an offset reducing amplifier followed by a digital integrator consisting of an analog-to-digital converter and a DSP, the sum of which comprises a long-term flux meter. The system disclosed by Eulenberg does not include description of a method or apparatus for determining position from the coil-integrator magnetic field measurement system and only claims the long-term integrator portion of the disclosure.
U.S. Pat. No. 5,453,686 to Anderson discloses a position measuring system using the same type of transmit waveform and position algorithm as disclosed in U.S. Pat. No. 4,849,692 to Blood but with the addition of a coil-integrator sensor means similar to that which is disclosed in the Eulenberg patent. The coil-integrator sensing means which is comparable to a well known flux gate magnetometer is well known in the art to produce results equivalent to a flux gate magnetometer when measuring transient magnetic events. The transmitting waveform disclosed in U.S. Pat. No. 4,849,692 to Blood is precisely the same as that which is used in the Anderson patent. The transmitting waveform disclosed in Blood ""692 and Anderson utilizes only half of the available transmit energy when the coil integrator is employed as the sensing element. While Anderson does not require compensation for the static portion of the Earth""s magnetic field, when the sensor coils are rotated in the Earth""s magnetic field, an undesired electromagnetic field is generated at the coil terminals and integrated. As a result, sensor offset errors occur due to dynamic sensor motion in a static magnetic field. The present invention contemplates reducing this type of error.
U.S. Pat. No. 5,767,669 to Hansen et al. discloses a system in which a triangular non-steady state transmit waveform is employed to overcome eddy current distortions that would otherwise be created by adjacent conductive metals. In one embodiment of the Hansen et al. patent, a transmit waveform is produced in such a manner that eddy current conditions in the conductive metal environment reach a steady state condition during both the rising and falling edges of the transmit waveform. The Hansen et al. patent also discloses numerous techniques for reducing the duration of either the rising and/or falling edges of the transmit waveform to increase the measurement rate. In all disclosed versions, the system requires that the integration reset and output digitization occur during transient conditions of the transmitted waveform, thereby requiring a high bandwidth signal chain. This mode of operation also requires extremely precise time synchronization between the transmitter and sensor signal processing. In motion capture applications, it is highly desirable to facilitate operation devoid of physical connections between the transmitter and signal processor so that the person who is performing the choreographed movements is unencumbered by attached cabling. Often, precision in measurements can be lost when attempting to synchronize when using a wireless configuration. The concept of time jitter is often encountered. Such time jitter results in reduction in synchronization precision which produces noise, offsets and other undesirable effects on system output. The present invention contemplates reducing these undesirable effects.
In the prior art, pulsed DC transmit waveforms are produced such that the falling edges of the X and Y axes overlap with the rising edges of the Y and Z axes, respectively, such that resetting the integration before the transient condition and integrating the coil output during the transient period produce a value that does not represent a useful quantity. Additionally, prior art systems require that the integrator be reset at the beginning of an X, Y, Z, OFF cycle and then read at the end of that cycle. The difference between the two values consists of the inherent drift of the integrator plus any output contribution due to motion in a static magnetic field. This difference is then divided proportionally according to time and subtracted from the measured values of X, Y and Z sensed fields. By contrast, in the preferred embodiment of the present invention, a pulsed DC transmit waveform is created that allows both the rising and falling edges of the transmitted waveform to be converted into useful signal.
The present invention relates to a method of measuring position and orientation with improved signal to noise ratio. The present invention includes the following interrelated objects, aspects and features:
(1) In a first aspect, the present invention is intended to be used to measure position and orientation in six degrees of freedom, namely, location in the three coordinate directions commonly referred to as the X, Y and Z axes, and orientation coordinates commonly described as azimuth, elevation and roll. In the preferred embodiment, a pulsed DC transmit waveform is created that allows both the rising and falling edges of the transmitted waveform to be converted into signal usable in performing position and orientation measurements.
(2) In the preferred embodiment, a symmetrical transmitting waveform is employed that includes distinct non-overlapping axes. One example of such a waveform is a square wave that includes extremely steep rising and falling edges. Signal processing is preferably accomplished in a manner in which the integrator is reset at the start of the rising edge transient period and the coil output signal is integrated throughout the rising edge transient and steady state periods. The integration result is measured at the end of the steady state period just before the waveform begins to fall. The integrator is then reset and the coil output signal is integrated throughout the transmitter falling edge transient and off periods, and the integration result is measured at the end of the off period.
(3) By making the duration of the first integration equal to the duration of the second integration errors in integration output due to constant offset sources are equal and of equal sign. The output components due to the transmitted magnetic field however are equal and of opposite sign. Thus, by subtracting the second integration result from the first integration result, the offset components cancel and the signal components add. As a result, a 2:1 signal gain over prior art measuring techniques is accomplished while simultaneously automatically canceling integrator offset errors.
(4) A further advantage of the inventive technique is that dynamic errors caused by moving the sensor coil in the static Earth""s field are reduced as the Nyquist sampling frequency is increased. When comparing the inventive method to the prior art, for equal eddy current settling steady state dwell times, the present invention achieves a 4:1 higher effective sampling rate. Thus, errors caused by motion of a sensor in the Earth""s static field are reduced by a factor of 4 as compared to the prior art.
(5) The aliasing frequency for external interference sources is also increased by a factor of 4 and interference components below this frequency are also 4 times smaller than would be the case in prior art systems. Low frequency noise components caused by amplifiers and other circuit elements are also reduced in the same manner and to the same degree.
(6) The inventive method represents a strong departure from the prior art relating to position and orientation measuring systems in that it is capable of utilizing all of the available energy in the transmit waveform, thereby providing a 2:1 signal to noise gain over prior art pulsed DC systems which utilize the same time rate of change sensing means. The inventive method does not require compensation due to the presence of the Earth""s magnetic field and also allows the critical integrator reset and sampling periods to be performed during the steady state portion of the transmitted waveform so that synchronization requirements are relaxed significantly. Post-processing computation is not required in order to remove integrator drift effects.
Accordingly, it is a first object of the present invention to provide a method of measuring position and orientation with improved signal to noise ratio.
It is a further object of the present invention to provide such a method having a significantly higher effective sampling rate over prior art systems.
It is a still further object of the present invention to provide such a method in which first and second integrations are conducted for equal time periods so that errors can be offset and thereby eliminated.
It is a still further object of the present invention to provide a method for quantitatively measuring the position and orientation of a sensor relative to a transmitter while avoiding compensation for the disadvantages of the static Earth""s field.
It is a yet further object of the present invention to provide such a method in which all critical control operations are conducted during the steady state of the transmitted waveform, thereby eliminating the potential effects of time jitter and other system non-linearities.
It is a still further object of the present method to improve dynamic performance when the sensor is rotated in the static Earth""s magnetic field.
It is a still further object of the present invention to provide such a method which improves low frequency noise immunity.
These and other objects, aspects and features of the present invention will be better understood from the following detailed description of the preferred embodiment when read in conjunction with the appended drawing figures.